Technical Field
The invention relates to biomolecules conjugated to graphene quantum dots, and in particular, to use of such biomolecule-graphene quantum dot conjugates as a fluorophore for imaging methods.
Description of the Related Art
Real-time tracking of fluorophore-tagged biomolecules is instrumental to reveal the dynamic cell functions at single cell or sub-cellular level. An ideal fluorophore should be conveniently excitable, bright, stable, equipped with chemical handles for readily conjugation with target molecules, biocompatible, and small enough to minimize physical hindrance.
Currently, organic dyes and fluorescent proteins are predominantly used for bio-imaging. They, however, intrinsically suffer from poor photo-stability problem which makes long-term imaging challenging because of fast photo-bleaching. In addition, labeling with fluorescent proteins involves non-trivial molecular biology processes including construction of chimeric plasmids and subsequent transfection in live cells. And the abundance of expressed chimeric fluorescent proteins is often low due to ineffective hijacking of the native genetic machinery and the damages or cytotoxicity caused by the transfection procedure.
Semiconductor quantum dots (QDs) have been regarded as the promising alternative to organic fluorophores because of their high brightness and photo-stability. They have been successfully employed for live-imaging of various cellular processes. But QDs are toxic due to leaching of heavy metal ions. And since they are much larger (typically >500 kDa) than a biomolecule, they may alter the function and trafficking of the target molecule, for example, steric hindrance introduced by such large tag may prevent the binding of the target molecule with its receptor. Also because of its large size, one QD carries multiple target molecules creating an artificial cluster which may lead to unphysiological consequences. Their proneness to aggregation and usually needed polymeric functional coating further exaggerates the aforementioned “size” issues.
Recently, graphene quantum dots (GQDs), which are individual single-atom-thick or a-few-atom-thick nanometer-sized planar sheet of graphitic carbon, have sparked significant excitement as a promising new class of fluorophores for bioimaging, owing to their interesting and tunable photoluminescence properties originated from quantum confinement, excellent photo-stability, biocompatibility, good water solubility, chemical inertness, small size, and low cost. Several groups have demonstrated that GQDs can be uptaken into live cells and remain fluorescent in various cellular locations without introducing apparent cytotoxity, indicating the bioimaging capability of GQDs. In a pioneer work, Dai and coworkers have showed that the PEG-modified nanographene oxide sheets (˜20 nm) functionalized with anti-CD20 can act as near-infrared fluorophores for selective recognition and imaging of CD20-expressing Raji B-cells (Liu Z, Robinson J T, Sun X M, Dai H J. PEGylated nanographene oxide for delivery of water-insoluble cancer drugs. J Am Chem Soc 2008, 130(33): 10876-10877).
Despite its highly anticipated potentials, GQDs have yet to be used to specifically label and track molecular targets involving in dynamic cellular processes in live cells.